Several mechanical systems provide for transverse needle swing in zigzag sewing machines. The transverse needle swing, i.e., from side to side, in the course of sewing a fabric, aids in sewing, particularly in sewing a seam or design in a fabric. The term fabric is used here to refer to any material that may sewn on a sewing machine and is not limited to a woven material including a warp and weft. Non-woven fabrics and sewable goods that are not fabrics at all, for example, plastics and leather, are within the scope of the term "fabric" as used here.
Conventional sewing machines produce zigzag stitching by using either a rotary hook system that provides a relatively wide transverse needle swing, for example, an amplitude of 9 millimeters between the extreme transverse positions of needle swing, or an oscillating hook system. In the oscillating hook system, the transverse swing has been limited, particularly in free-aim consumer sewing machines, to an amplitude of about five millimeters. The invention described below particularly pertains to oscillating hook system sewing machines providing a wider traverse needle swing without loss of the higher stitch quality produced by the oscillating hook mechanism.
FIGS. 1(a) and 1(b) are schematic front and rear views of a conventional oscillating hook sewing machine drive system in a free-arm sewing machine. The front view of FIG. 1 (a) is cut away to show thread, needle, and bobbin relationships and is the view that a person operating the sewing machine would see if the free arm covering were transparent. FIG. 1(b) is a rear view of the oscillating hook system showing a driving mechanism for driving the oscillating hook. FIG. 1(a) illustrates a "front loading" configuration in which a bobbin is inserted into the sewing machine along a direction perpendicular to the plane of FIG. 1 (a). However, other arrangements, to which the invention likewise applies, are known. For example, the bobbin may be loaded from the side, i.e., along a direction parallel to the plane of FIG. 1(a). Still other bobbin loading positions may be employed. Although the invention, described below, is not limited to a particular bobbin loading configuration, the illustrated front loading bobbin arrangement is preferred.
In all figures, like elements are given the same reference number. In FIG. 1(a), a needle 1 engages a needle thread 2. Thread is used in a general sense here and means any multiple strand or single strand filament that can be used to join two pieces of fabric or to decorate a fabric. The needle thread 2 passes through a hole or eye of the needle 1. The needle 1 is driven by a motor, not shown, in an oscillating motion in a generally linear direction, usually vertical, subject to the transverse swing of the zigzag stitch. The drive from the motor may be indirect, through gears, belts, or the like. The needle, in its linear, up and down motion, passes through a hole 3 in a needle plate 4 that supports a fabric (not shown) being sewn. The needle thread 2 is maneuvered, as described below, by an oscillating hook 5. A second thread, a bobbin thread 6 is supplied from a bobbin 7 that is coaxial with the hook 5.
As shown in FIG. 1(b), the oscillating hook 5 includes a shaft with a toothed gear, i.e., a pinion, 20 that is driven by a rack 21 having teeth that engage the teeth of the gear 20. The rack 21 is pivotally connected to a link 22 at a pin 23 that permits rotational movement of the rack 21 relative to the link 22. The opposite end of the link 22 is pivotally pinned by a pin 25 to a driven rotating wheel 24 near the periphery of the wheel. The wheel 24 is rotationally driven by a motor (not shown), directly or through gears, belts, or another transmission mechanism. Preferably, the same motor drives the wheel 24 and the needle 1. Separate motor drives may be used provided they are maintained in synchronization.
In the conventional mechanism shown in FIG. 1(b), as the wheel 24 rotates, the link 22 is caused to move transversely, i.e., from left to right, in, essentially, simple harmonic motion. The rack 21 likewise, to the extent its motion is only planar, moves in nearly simple harmonic motion. Because of the presence of the two separate elements, the link 22 and the rack 21 connected at the pin 23, the actual motion of the rack 21 deviates from simple harmonic motion. The deviation is relatively small and of the second order. For purposes of this disclosure, that minor deviation is considered to be insignificant and the term "simple harmonic motion" is used to encompass the repetitive cyclic transverse movement of the rack 21 between extreme left and extreme right positions. As understood from the basic definition of simple harmonic motion, at two points in each cycle, a moving element has a zero velocity. The speed of the element increases from the two zero velocity points, e.g., the most extreme right and left positions of the rack 21 in FIG. 1(a), to a maximum velocity at a position intermediate the two extreme positions.
The stitching produced by the rotation of the wheel 24 in driving the hook 5 through the engagement of the gear 20 and the rack 21 is described with respect to FIGS. 2(a)-2(d). FIGS. 2(a)-2(d) show four sequential positions of the oscillating hook 5. In FIG. 2(a), the oscillating hook 5 is in an extreme position after rotating counterclockwise and is at zero velocity. This extreme position is approximately synchronized with the lowest position of the needle 1. The needle thread 2 extends from a source of thread, e.g., a spool, and other mechanisms (not shown), downward and through the eye of the needle 2. The other mechanisms include a means of adjusting the tension of the thread and a take-up lever, known elements of conventional sewing machines located above the needle plate 4. The thread 2 extends beneath the needle plate 4, through the hole 3, loops, and extends back and out through the hole 3 in the needle plate 4 on the top side of the needle plate 4. A needle thread loop 8 is located beneath the needle plate 4. The bobbin thread 6 extending from the bobbin 7 underneath the needle plate 4 also passes through the hole 3 and extends on the top side of the needle plate 4.
As shown in FIG. 2(b), as the needle 1 begins to rise, the hook 5 begins to turn clockwise, a hook finger 9 on the end of the hook 5 passes through and engages the needle thread loop 8 at a position spaced from the needle 1. The hook 5 continues to rotate in a clockwise direction, as shown in FIG. 2(c), increasing the size of the needle thread loop 8 as the hook finger 9 pulls on the needle thread 2. The needle thread look 8 is pulled almost completely around the bobbin 7. The needle 1 is in approximately its highest position in FIG. 2(c). At that position, the take-up lever (not shown) above the needle plate 4 is beginning to rise, pulling on the needle thread 6.
The take-up lever continues to rise and, as shown in FIG. 2(d), the needle thread 2, formerly wrapped around the bobbin 7, slips off the hook finger 9, surrounding the bobbin thread 6. The needle thread loop 8 is pulled closed, gripping the bobbin thread 6, and brought tight against the bottom surface of the fabric being sewn, so that a locked stitch is formed. The needle 1 finishes its rise and then moves downward to form another needle thread loop 8 for the next stitch. In the meantime, the hook 5 rotates counterclockwise, starting from a zero velocity at the maximum clockwise position, moving to its maximum counterclockwise position, i.e., the position shown in FIG. 2(a). In this arrangement, the hook 5 reaches its most counterclockwise position at approximately the same time the needle 1 reaches its lowest position. Thereafter, the stitching cycle as described is repeated. The position and the timing of the motion reversals at the extreme rotational positions of the hook 5 relative to the needle 1, and the timing of the engagement by the hook finger 9 of the needle thread loop 8 are critical to proper stitching by a sewing machine incorporating the oscillating hook mechanism.
The movement of the oscillating hook in the sequence shown in FIGS. 2(a)-2(d) is illustrative, not limiting. For example, operation in a mirror image configuration is possible and the invention, described below, is not limited to a particular orientation of the hook finger 9.
In the sequence of motions illustrated in FIGS. 2(a)-2(d), it is apparent that the needle 1 is moving upward at the time the hook finger 9 moves toward the needle 1 to engage the needle thread loop 8. The position along the length of the shank of the needle 1 at which the hook can reliably engage the needle thread loop 8 is limited by the vertical extent of the needle thread loop that is formed behind the needle 1 as the needle rises from its lowest point towards its highest point. Those of skill in the art refer to adjusting the synchronization of the needle, needle thread loop, and the hook finger as setting the hook timing.
To produce design or zigzag stitching on a fabric, a sewing machine may control the needle 1 to penetrate the fabric at a center position aligned with a vertical axis of the needle or at offset positions on either side of the center position. When the needle penetrates the fabric to the left of the center position of the needle, with respect to the description of FIGS. 2(a) and 2(b), the hook finger 9 engages the needle thread 2 at a time earlier in the stitch cycle and at a position higher relative to the needle shank than when the needle penetrates the sewing material at a center position. Likewise, when the needle penetrates the fabric at a position to the right of the center position of the needle 1, the hook finger 9 engages the needle thread loop 8 at a position that is lower relative to the shank of the needle. The relationship between the positions of the needle 1 and the hook finger 9 at the moment of engagement of the needle thread loop 8 for left of center, center, and right of center stitch positions are illustrated in FIGS. 3(a), 3(b), and 3(c), respectively. For simplicity, the needle thread 2 and the bobbin thread 6 are omitted from these figures. FIG. 3(a) demonstrates that the hook finger 9 is at a higher position relative to the shank of the needle 1 in the left of center position than for the other two positions. Likewise, the hook finger 9 is lower relative to the shank of the needle 1 in the rightmost position of the needle in FIG. 3(c) as compared to the other two illustrated positions.
FIGS. 3(a)-3(c) show that as the left-right swing of the needle increases in amplitude, the difference in hooking position relative to the shank of the needle increases. However, the distance along the needle shank for which the needle thread loop 8 is reliably engaged by the hook finger 9 is limited. Thus, the amplitude of the left-right needle swing in an oscillating hook sewing machine is limited to ensure proper sewing results. For example, in conventional consumer sewing machines with a free-arm construction and oscillating hook, the amplitude of the needle swing is limited to approximately 5 millimeters.
If the rotational speed of the oscillating hook were proportionally increased as the transverse needle swing amplitude is increased, the hooking position at which the hook finger reliably engages the needle thread loop 8 relative to the shank of the needle 1 could be maintained within the range depicted in FIGS. 3(a)-3(c). The rotational velocity of the hook finger 9 can be increased without changing the angular velocity of the hook shaft or changing the timing of the rotation reversals of the hook 5 by increasing the diameter of the hook 5. However, increasing the diameter of the hook would undesirably increase the size of the free arm of a free-arm sewing machine for domestic use.
In a rotary hook mechanism, higher hook speed and, consequently, wider transverse needle swing may be achieved because the rotary hook makes two continuous complete rotations for each stitch cycle rather than the clockwise and counterclockwise oscillation of the oscillating hook for each stitch cycle. The oscillating hook provides greater stitch precision than the rotary hook because the oscillating hook stops and changes direction twice during each stitching cycle. With each change in rotational direction of the oscillating hook, the needle thread passes with no resistance between the hook finger 9 and the hook driving mechanism, minimizing the effects on manual thread tension adjustments relative to changing sewing conditions. By contrast, in the rotary hook system, the needle thread must be pulled past the hook driving mechanism for each stitch. Thus, the needle thread tension requirements change with changing sewing conditions, requiring more frequent manual tension adjustments. Thus, although the rotary hook system can sew faster, thread tension settings are more sensitive to sewing conditions with a rotary hook system than with an oscillating hook system. Since the oscillating hook mechanism provides more consistent stitch quality with less required operator control, it is particularly desirable to provide an improvement in the oscillating hook mechanism that provides for wider amplitude transverse needle swing.